Field of Invention
The present application provides a method for determining the flow rate of an aqueous phase of a multiphase flow. In particular the method is useful for determining the flow rate of the aqueous or "water" phase of a two phase flow in a hydrocarbon production well. In such a case, the other phase is an oil phase which is immiscible with the water phase which may contain dissolved substances as well as water.
A common method of monitoring the flow of fluids is to introduce some chemical or physical identifier, known as tracer, into the flow and then to monitor the passage of that tracer by means of some measurement. Examples include the introduction of saline solution into water flows and using electrical conductivity as a monitor, or introducing radioactive materials and using radiation detectors to track the progress of the fluid. A tool, called a Tracer Ejector Tool "TET", for injecting tracers into wellbore flows is commercialised by the company Schlumberger, frequently for monitoring the flows of water in injection wells. This tool and the methods of acquiring and interpreting the data are disclosed in U.S. Pat. Nos. 4,166,215 and 4,166,216. These patents relate to a tracer-survey method and apparatus for obtaining measurements representative of the dynamic flow characteristics of the continuous-phase present at one or more depth locations in a production well containing a lighter discontinuous-phase wellbore fluid. Minor amounts of suitable radioactive tracer are periodically discharged into the fluids at the selected depth location in the production well. Thereafter, by simultaneously monitoring the level of radioactivity present in the wellbore fluids above and below that depth location, measurements are obtained which are representative of one or more dynamic flow characteristics of the heavier continuous-phase at that depth location in the wellbore. These measurements are based on the travel time of the tracer from the location where it is discharged in the flow to another location where it is detected. However, the interpretation of the results has proved to be difficult due to the "smearing out" of the signal caused by the extensive mixing generated by the passage of the bubbles or droplets of the dispersed phase. Due to the large width of the signals obtained, it is difficult to locate precisely the peaks of the signals and therefore large errors are incurred by considering the time difference in peak arrival as representing the transit velocity of the flow.
A further method of monitoring fluid flow is disclosed in U.S. Pat. No. 4,233,508 granted to Arnold. In this case, instead of mixing a tracer with the fluid being monitored, the fluid is irradiated with neutrons. The patented method comprises a method of determining the flow characteristics of injection water in a perforated, cased hole. The injection water is irradiated with neutrons such that oxygen atoms in the water are transformed into radioactive nitrogen atoms. These are unstable and decay by emitting .gamma. radiation which is detected at various locations relative to the neutron activation location to determine the movement of the injection water in conjunction with separate measurements taken with a flow meter.